Apparatus for imaging metrology

ABSTRACT

This invention is an apparatus for imaging metrology, which in particular embodiments may be integrated with a processor station such that a metrology station is apart from but coupled to a process station. The metrology station is provided with a first imaging camera with a first field of view containing the measurement region. Alternate embodiments include a second imaging camera with a second field of view. Preferred embodiments comprise a broadband ultraviolet light source, although other embodiments may have a visible or near infrared light source of broad or narrow optical bandwidth. Embodiments including a broad bandwidth source typically include a spectrograph, or an imaging spectrograph. Particular embodiments may include curved, reflective optics or a measurement region wetted by a liquid. In a typical embodiment, the metrology station and the measurement region are configured to have 4 degrees of freedom of movement relative to each other.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalApplication Ser. No. 60/125,462, filed Mar. 22, 1999 , and is acontinuation-in-part of U.S. Utility Application Ser. No. 09/495,821,filed Feb. 1, 2000, now issued U.S. Utility Pat. No. 6,690,473, all ofwhich are incorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of optical metrology in general, andto in-line thin-film reflectometry and profilometry for semiconductorwafers in particular.

2. Description of Related Art

A trend towards smaller critical dimension sizes in integrated circuits(IC) drives advances in technology for semiconductor capital equipment.Both technical factors, such as the ratio of the critical dimension sizeto the wavelengths of light used by fabrication device components, andwell known economic factors, such as wafer throughput, Cost-Of-Ownership(COO) and Overall Equipment Effectiveness (OEE) are critical.

In IC fabrication, hundreds of process steps are necessary. During someof these steps, successive layers of materials are deposited on asubstrate. Subsequently, Chemical Mechanical Polishing (CMP) is oftenused to make a film layer planar to high degree of precision. After aCMP process step, the thickness of the remaining film may be determinedto verify that it is within desired tolerances.

Optical methods are commonly used to determine the thickness of thinfilms since light is generally non-destructive and non-invasive.Measured optical properties of the surface or measured wave-opticseffects due to the interaction of light with thin films residing on thewafer yield desired information, such as film thickness. Thus, ascritical dimensions on the wafer are reduced, there is a need foradvances in optical metrology to obtain required precision and accuracy.

Technical requirements of precision and accuracy must be consonant witheconomic requirements. Fabrication machines must process wafers at arapid rate with high uniformity and high reliability in addition to highprecision. Since the fabrication must take place in a strictlycontrolled environment, the size of the machine is also an importantfactor. Easy operation is also important, despite the complexity of theprocessing and measurements. Performance in terms of these and othereconomic factors can be expressed through figures- of-merit such as COOand OEE.

Wafer metrology art comprises mostly “metrology mainframe” devices,which are devices only partially integrated with an IC fabrication line.There are at least two significant problems associated with partiallyintegrated or non-integrated metrology control. First, waiting for testmeasurements from metrology mainframe systems to confirm the resultsfrom each process step is inherently inefficient. Second, with apartially integrated or non-integrated unit, process engineers facedifficulties in achieving and maintaining optimal process parametersonce they have the measurement information.

These and other problems associated with off-line metrology result ingrowing need for integrated (in-line) metrology in IC wafer fabrication.With in-line devices, the metrology apparatus is physically placedwithin the process equipment itself This enables a substantial reductionin times required to perform metrology measurements and shortensfeedback or feedforward times between the metrology system and theprocess controls. By measuring critical parameters as each wafer isprocessed, the process equipment has information on the most recentlyprocessed wafer without stopping production. This results in goodwafer-to-wafer control. Integrated metrology also significantly reducesoperating costs by reducing the requirement for expensive test wafers,speeding up process qualifications and maintenance schedules, andprovides an overall reduction in scrap wafers.

Related art in integrated thin-film metrology is limited regardingcombining precise and accurate thin-film thickness measurements whilemeeting the other requirements of the semiconductor industry. Typically,related art in-line devices are limited to measurements of films ofabout 80 nm thickness. However, there is a need in the industry tomeasure film thickness of only a few tens of nanometers. Further,related art in-line devices are limited in their ability to make rapid,successive measurements over the totality of a wafer's surface.

What is needed is an imaging metrology system with rapid optical accessto the entirety of a wafer surface. From the foregoing, it can bereadily appreciated that many processes used in microelectronicsmanufacturing could benefit from integrated metrology, including but notlimited to CMP, plasma etching, chemical vapor deposition, andlithography.

SUMMARY OF INVENTION

This invention is an apparatus for imaging metrology. One object is tointegrate an imaging metrology station with a processor station suchthat the metrology station is apart from but coupled to the processstation.

In one embodiment, a metrology device is provided with a first imagingcamera with a first field of view containing the measurement region.Alternate embodiments include a second imaging camera with a secondfield of view. Preferred embodiments comprise a broadband ultravioletlight source, although other embodiments may have a visible or nearinfrared light source of broad or narrow optical bandwidth. Embodimentsincluding a broad bandwidth source typically include a spectrograph, oran imaging spectrograph. Particular embodiments may include curved,reflective optics or a measurement region wetted by a liquid. In atypical embodiment, the metrology station and the measurement region areconfigured to have 4 degrees of freedom of movement relative to eachother.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a hardware overview for one embodiment of the wafermetrology device.

FIG. 2 illustrates a prior art device with a single large window.

FIG. 3 shows an exemplary reference reflector embodiment.

FIG. 4 shows an exemplary embodiment of the wafer aligner.

FIG. 5 illustrates calibration of the wafer aligner.

FIG. 6 illustrates improved accuracy of wafer alignment.

FIG. 7 illustrates the use of a large Field-Of-View (LFOV) camera and asmall Field-Of-View (SFOV) camera to avoid groping in the process oflocating a particular region of a wafer.

FIG. 8 illustrates the advantage of using of the LFOV camera to enableeasy die size determination during training.

FIG. 9 illustrates one embodiment of an integrated metrology apparatus.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of an imaging metrology device. InFIG. 1, reflectometer assembly 100, vacuum chuck 101, vacuum chucksymmetry axis 102, light source fiber 103, first beam splitter 104,second beam splitter 105, semiconductor wafer 110, measurement region111, window 120, collimator 130, relay optics. 135, first imagingoptical assembly 137, second imaging optical assembly 138, third imagingoptical assembly 139, spectrographs (including calibration filters) 140and 141, spectrograph fiber optic 145, pinhole mirrors 146, largefield-of-view camera 150, small field-of-view camera 160, auto-focussingobjective lens assembly 190, first optics breadboard 195, and secondoptics breadboard 197 are shown. The embodiment shown in FIG. 1 may beintegrated as a subsystem into a process device (not shown) or in otherembodiments may be a stand-alone mainframe. Other embodiments mayinclude other diagnostic assemblies without departing from theinvention, as described below.

In FIG. 1, semiconductor wafer 110 is coupled to vacuum chuck 101, whosecenter-of-mass is fixed relative to the laboratory and the semiconductorwafer coupled to it. However, rotation of the vacuum chuck about thevacuum chuck symmetry axis 102 is allowed. Reflectometer assembly 100comprises window 120 and first and second optics breadboards 195 and197, respectively. First optics breadboard 195 is free to translatealong the y axis, and may be driven by a direct-drive actuator in aparticular embodiment. Second optics breadboard 197 is coupled to thefirst optics breadboard, however, the second optics breadboard is freeto translate relative to the first optics breadboard along the x axis.Objective lens assembly 190 is attached to the second optics breadboard,however, it is free to translate along the z axis as controlled by anauto-focus system known to the art. Thus, the embodiment shown in FIG. 1has four degrees of freedom of movement: translation along the (x, y, z)axes, and rotation of the vacuum chuck about the vacuum chuck symmetryaxis. This allows rapid optical access to the entirety of the wafersurface.

In the embodiment shown in FIG. 1, all optical elements except those onsecond optics breadboard 197 are coupled to and fixed relative to firstoptics breadboard 195. Objective lens assembly 190, are coupled tosecond optics breadboard 197. Thus, the objective lens assembly is freeto translate along the x axis. In addition, the objective lens assemblymay be focussed on semiconductor wafer 110 by translation along the zaxis. Note that translations of the first and second optics breadboardsalong the x axis and y axis allow access to the full wafer surface.Rotation of the wafer coupled to the vacuum chuck may be used incombination with translations of the first and second optics breadboardsalong the x and y axes to allow more rapid measurement access over theentire surface of the semiconductor wafer or to eliminate obstructions.Complete coverage of a 200 mm diameter wafer is possible withstraightforward scaling to 300 mm and larger diameter wafers.

In FIG. 1, vacuum chuck 101 not only holds wafer 110 but also flattensit. This enables more accurate auto-focus, and reduces measurementerrors and uncertainties due to wafer tilt and associated variations inoptical path length.

In the embodiment of FIG. 1 and in alternate embodiments, opticsbreadboards 195, 197 may be positioned with a direct drive motor/leadscrew. In a preferred embodiment, components of a motor are mounteddirectly on a lead screw shaft according to well-understood mechanicaltechniques. With a direct drive motor/lead screw, coupling elements areeliminated resulting in a more compact drive mechanism with highstiffness in torsion that enables precision positioning of thebreadboards relative to wafer 110.

The wavelength range for illumination and collection may be in theultraviolet (UV) or the visible or the near infrared (NIR) in differentembodiments. In the particular embodiment in FIG. 1, reflectometerassembly 100 comprises a broadband (UV, visible, NIR) reflectometermeasurement system. Other embodiments may be narrowband or may compriseinstruments other than a reflectometer. In FIG. 1, an illuminating lightsource (not shown) may be a UV Xenon lamp, fiber-coupled to the systemshown in FIG. 1 via source fiber 103. Alternate embodiments have atungsten lamp or a deuterium lamp or a xenon lamp. Relay optics 135transfer collimated light from lens assembly 130 to beam splitter 104.The light transmitted directly through the beam splitter from the sourcefiber is referred to as the monitor beam. The monitor beam does notinteract with measurement region 111. The portion of the illuminationthat the beam splitter directs toward the wafer is referred to as themeasurement beam. The measurement beam reflects from the surface of thewafer, where its spectrum is modified by the presence of thin films onthe wafer.

Following reflection from the wafer, the measurement beam returns to thebeam splitter, and passes to several relay mirrors 135. First imagingoptical assembly 137 focuses the measurement beam onto pinhole mirror146. The light falling on a pin hole aperture in the pin-hole mirrorpasses into spectrograph fiber 145, which conveys it to spectrograph140. The resulting spectrum is a primary source of information about thefilms on the wafer. Other embodiments may image a portion of the wafersurface onto a spectrograph slit, thereby collecting data along a lineon the wafer surface rather than a point.

Referring again to FIG. 1, the monitor beam follows a similar butdistinct path through another pinhole mirror 146 and spectrograph fiber145 to spectrograph 141. The measured monitor spectrum is indicative ofthe illumination and optical components, and may be used to correct themeasurement of film properties for instrument characteristics.

As described above, the relative spectral content of both the incidentand reflected light from semiconductor wafer 110 is measured. Thethickness of thin-films deposited on the measurement region 110 can thenbe determined from the reflected measurement beam and incident monitorbeam light by principles well known in the art. Many data reductionmethods are applicable.

The embodiment illustrated in FIG. 1 has several advantages. First, asdescribed above, an entire wafer surface can be quickly accessed. Inaddition, scanning with relay mirrors is employed in only one spatialdimension. If the light beams reflected from the relay mirrors wereperfectly collimated and aligned, scanning would have no deleteriouseffects on the performance of the system. However, the beams cannot beperfectly collimated and perfect alignment is unattainable in practice.Therefore, it would be preferable to scan the objective with respect tothe rest of the optics as little as possible. In the embodiment shown inFIG. 1 the majority of the optics scan in one dimension on the firstoptics breadboard, and the rest of the optics scan in two dimensionswith respect to a laboratory-fixed coordinate system, but only onedimension (X) with respect to the first optics breadboard. Thus, therelay scan length is no more than one wafer diameter. In related artdevices, the optics are fixed, and the objective scans in twodimensions, requiring a scan length of up to two wafer diameters.

A further advantage of the embodiments shown in FIG. 1 is that theoptical path length remains constant, regardless of scan position. Thus,if the object is treated as a focal point, with a specular reflectionfrom the surface of the wafer, the amount of beam spreading does notchange. In related art devices, spatial scanning over a wafer surfacechanges the total optical path length, and thus the amount of beamspreading suffered by a collimated beam.

According to an aspect of this invention, locating a particular regionof a wafer for measurement is achieved by imaging at least onefield-of-view of a surface of the wafer. In the embodiment shown in FIG.1, reflectometer assembly 100 measures selected regions of semiconductorwafer 110 as located and identified by imaging cameras. Largefield-of-view camera 150 and small field-of-view camera 160 image thewafer surface with an approximately 20 mm×27 mm and an approximately 1mm×1.3 mm field-of-view, respectively. In FIG. 1, a portion of themeasurement beam reflected by pinhole mirror 146 is refocused onto smallfield-of-view (SFOV) camera 160. The resulting image is indicative ofpatterns on semiconductor wafer 110. The pinhole itself is also imagedonto the SFOV as a dark spot superimposed on the image of the wafer'spatterns. This dark spot indicates the precise location where thereflectometer measurement is made with respect to the patterns on thewafer. Alternate embodiments may include a Fresnel lens and abeamsplitter plate or utilize dark-field illumination.

FIG. 7 illustrates the use of a large Field-Of-View (LFOV) camera and asmall Field-Of-View (SFOV) camera to image a wafer surface and avoidgroping in the process of locating a particular region of a wafer. InFIG. 7, die 700, LFOV 702, SFOV 703, LFOV pattern 704, and SFOV pattern701 are shown.

LFOV 702 is generally larger than die 700, and much larger than theuncertainty in the location of the center of the wafer. Thus, it can bemoved to a location where it will certainly find LFOV pattern 704 on adie of a randomly oriented wafer. Once the LFOV pattern has been found,the system has much better knowledge of both the orientation of thewafer and the location of its center. Thus it is able to position theSFOV 703 over the SFOV pattern 701 without groping. This process has adeterministic time that is much shorter than the worst-case scenario forgroping with just a SFOV, or than the time for physically aligning thewafer.

FIG. 8 illustrates the advantage of using of the LFOV camera to enableeasy die size determination during training. In FIG. 8, dies 800,inter-die streets 840, inter-die alleys 850, die features 805 a-c, largefield of view 802, small field of view patterns 803, 804 and 801, andmeasurement site 806 are shown.

For training purposes, operators find it advantageous to view the waferright side up, and moreover to orient the wafer so that inter-diestreets 840 and alleys 850 appear vertical and horizontal, as shown inFIG. 8. However, such an orientation of the wafer is not necessary andother orientations are possible in alternative embodiments. An initialrough estimate of die size can be made from three occurrences of a diefeature, eg. 805 a-c, selected by the operator on three different dies.The system can then use pattern recognition and the LFOV and/or SFOVcameras to obtain a very accurate determination of die size by locatingLFOV and/or SFOV patterns, 804 and/or 801, on various dies on the wafer.With this method, it is not necessary for the operator to know the diesize a priori.

Another advantage of the LFOV camera is ease of training human operatorsto correlate measurement sites and patterns in the SFOV with theposition on the wafer. Ideally, the large field of view covers a wholedie, as shown in FIG. 8. Using large field-of-view 802, an operator canselect the region of the die 800 to view with SFOV 103. This is similarto using a state map to navigate to a particular city. Once the SFOV hasbeen properly positioned, the operator can very precisely select SFOVpattern 801 and the measurement site 806. This is similar to finding thecorrect intersection on a city map.

In a preferred embodiment, there may be a multiplicity of measurementsites within a die. In such cases, different sites may have different‘stacks’ of layers that are to be measured. The thickness algorithm,i.e., the parametric minimization of a cost function as discussed inU.S. Provisional Application Ser. No. 60/125,462, generally needs tohave a priori information, the algorithm recipe, about each stack thatis measured. In cases where there are multiple sites per die withdifferent stacks, the system must either use multiple algorithm recipes,or have a general algorithm recipe to accommodate the different stacks.

The reflectometer shown in FIG. 1 is included for purposes ofillustration and not limitation. Alternate embodiments of the imagingmetrology device comprise other metrology systems, including acousticsystems. Such alternate metrology system may be coupled to the sameoptics breadboards shown in FIG. 1 for reflectometer system 100.Particular embodiments may also require the use of flexural bearings forsmooth repeatable motion on micrometer or sub-micrometer scales.

Exemplary alternative embodiments include a profilometer to determineamounts of recess, dishing, or other departures from planarity of awafer surface and a profilometer in combination with a reflectometer. Indifferent embodiments, a profilometer may be an acoustic profilometer oran optical profilometer. A particular embodiment of an opticalprofilometer may use the auto-focus system described in U.S. ProvisionalApplication Ser. No. 60/125,462, to determine a relative profile of awafer surface. The auto-focus system is inherently sensitive to theprofile of the wafer surface since departures from planarity of thewafer surface will cause differences in the focussing of light raysreflected from the wafer surface.

Other embodiments of this wafer metrology device may include anellipsometer or high-contrast imaging microscopes. Particularembodiments may utilize aspects of differential interference contrast(DIC) techniques. Polarization techniques may be incorporated to inferquantitative information about the wafer surface according to techniqueswell known in the art. In particular embodiments, an integratedinterferometer, and imaging spectrograph may be used to simultaneouslydetermine the wafer surface's profile and material content. Preferredembodiments further comprise motion control systems, image patternrecognition systems, and software to determine the quantities ofinterest from measured data. These elements are well-known in the art.

It is noteworthy that in the embodiment shown in FIG. 1, semiconductorwafer 110 is located above reflectometer assembly 100. In alternativeembodiments, the semiconductor wafer may be held in a pool of waterabove or below the optical system. With the wafer below the optics, thesystem may be configured to ‘look’ down instead of up. This wouldnecessitate differences from FIG. 1 in the handling of the wafer, whichwould have its IC device side up. In such alternate embodiments, eitherthe optical system (including a main window) may be lowered toward thesemiconductor wafer or the semiconductor wafer may be raised toward theoptical system. Such an alternate embodiment would be a 180 degreesrotation of the system about a horizontal axis, as compared to FIG. 1.General rotations of the system relative to the configuration shown inFIG. 1 are also possible, e.g., 90 degrees. The main impact of suchrotations is on the wafer handling techniques.

In particular alternative embodiments of the invention, there may be nowater in the measurement path. That is, the instrument is ‘dry’. In suchembodiments, the orientation of the instrument relative to thelaboratory may be arbitrary. For example, the embodiment of FIG. 1 couldbe operated on its side or upside down. While redesign of some ofoptical components might be preferred in such cases, it would not benecessary.

As can be appreciated by the skilled person, many other optical layoutsthan that shown in FIG. 1 are also possible without departing from theinvention, including embodiments using substantially all reflectiveoptics including curved reflectors. As one skilled in the art willrecognize, the use of reflective optics has several advantages includingminimizing Fresnel reflections, and chromatic bandwidth limitation andaberrations. In certain embodiments, however, refractive opticalcomponents are preferred since reflective optics may introduceconstraints on aperture and geometry. In such embodiments, the opticalsystem is color-corrected and if the semiconductor wafer is immersed inwater, the water is considered as an optical component.

FIG. 2 illustrates a prior art device with a single large window fixedrelative to the laboratory. In FIG. 2, wafer 200, water surface 201,containment wall 203, objective lens assembly 207, beam splitter 235,relay optic 237, and window 202 are shown. It is noteworthy that thisprior art device utilizes a single large window 202. For accuratemeasurements, window 202 must be of optical quality. Due to the size ofthe window, this can lead to considerable expense.

To achieve accurate measurement results when a data reduction method,preferred embodiments use a reference reflector to collect data allowinga correction for slowly varying characteristics of the measurementsystem in a data reduction method. FIG. 3 shows an exemplary embodimentwith a reference reflector. In FIG. 3, wafer 300, window 302, referencereflector 309, reference volume walls 310, reference volume 311, mainvolume of water 301, objective lens assembly 307 and relay optics 335are shown.

In FIG. 3, reference volume walls 310 separate the reference volume 311from the main volume of water 301. Reference volume 311 may be filledwith air, water, or other suitable substances. It is preferred that thereflectivity of reference reflector 309 is very stable over time. Thedistance between window 302 and the reference reflector can be adjustedif volume 311 is not filled with water, to put the reflector in focuswhen the objective lens assembly 307 is the same distance below thewindow 302, as when the wafer is in focus. In a preferred embodiment,the volume is filled with an inert solid, and the height of thereflective surface above the window 302 is adjusted appropriately.

Reference reflector 309 may be of silicon, fused silica, chromium or anyother inert material. It may comprise layers of deposited material on asubstrate to achieve mechanical and optical stability. In a preferredembodiment, the reference reflector comprises a fused silica substratewith a chromium film on a top surface. An alternative embodiment of thereference reflector uses silicon with a reflective oxide layer on alower surface.

Referring to FIG. 3, reference reflector 309, reference volume walls 310and the window 302 may be assembled in differing ways. In a preferredembodiment, the reflector and window are hermetically sealed to thewindow. In an alternative embodiment, the reference reflector, referencevolume walls and the window are held together with a polymer adhesive,e.g., epoxy or super glue. In other embodiments, volume 309 is notsealed off from main volume 301. The components are either bondedtogether or held in place mechanically, for example with stops andsprings. The reference volume is sealed in order to preserve thereflectance of the reflector, i.e., to avoid it getting dirty orcorroded due to materials introduced into the bath, e.g., CMP slurry.Sealing volume 311 avoids the problem of breaks or leaks caused bydifferent thermal expansion coefficients, either during operation orshipping.

In preferred embodiments, reference reflector 309 is placed in aposition where the objective lens assembly 307 can have direct access toit. Preferably, the objective lens assembly can scan in at least onedimension, and move to the location of the reference reflector.

However, in embodiments where the wafer scans over the objective, thereference reflector may do so as well. While a preferred embodiment hasthe wafer above the objective lens assembly as illustrated in FIG. 3,alternate embodiments may have the objective lens assembly above thewafer, or at an arbitrary inclination.

According to aspects of this wafer metrology device described above, areference spectrum from the reference reflector 309 is collectedperiodically. Following collection of a reference spectrum a datareduction algorithm utilizing the reference spectrum is used tocalculate film thickness from spectra collected from wafer 300.Preferably, a reference spectrum is collected every time just prior to awafer measurement. There are numerous ways to include the referencespectrum from the reference reflector into a data-reduction algorithm.In one embodiment, every spectrum from the wafer is normalized with themost recently measured reference spectrum from the reference reflector.

Calibration of the measurement apparatus may utilize a calibration waferand the spectrum collected from it. Calibration adjusts the algorithmdescribed above so that it gives the correct answer for the calibrationwafer. The reference spectrum should be used by the algorithm atcalibration in the same way that it is used during measurements ofwafers, so that any changes in the system between the last calibrationand the current measurement will not affect the results of thealgorithm.

As described, embodiments of this wafer metrology device (see FIG. 1 andFIG. 3) may include a reference reflector and dual spectrographs. Theprimary data for the measurement is the spectrum S, which is thesystem's output representing reflection from the sample under test. Inaddition to the properties of the sample, S depends on thecharacteristics of the broadband (UV, visible, NIR) illumination, theoptical system, detectors and digitizers and other elements thatcomprise a measurement system. Such measurement system characteristicsobscure information about the sample. Thus, an accurate measurement offilm thickness should remove their effects.

Some characteristics of a measurement system change significantly withtime, and others may be substantially constant. In a preferredembodiment of this invention, an arc lamp is the light source.Flickering of the arc in its housing produces very fast changes. Bendingor flexing of source fiber 103 (see FIG. 1) and changing an optical pathlength due to scanning may give rise to fast changes. Aging of the lampmay produce slow changes.

According to aspects of this invention, dual spectrographs may collecttwo spectra essentially simultaneously, a reflection spectrum from thesample under test and the monitor spectrum that does not interact withthe sample under test, as shown in FIG. 1.

In FIG. 1 the sample under test is the semiconductor wafer 110. It isnoteworthy that the sample may also be the reference reflector or thecalibration reflector, as discussed above. From FIG. 1, the optical pathfor light determining the monitor spectrum may be similar to the opticalpath for the light determining the measurement spectrum, except fortransit to and from measurement region 111. A preferred embodiment ofthe two beams is shown in FIG. 1. In preferred embodiments, theillumination source may be identical for both beams.

In FIG. 1, a beam splitter divides the reflected beam from the monitorbeam, which proceeds straight through the beam splitter to thespectrograph 141. The reflected beam proceeds from the beam splitter,through the objective and to the sample, back through the objective andbeam splitter to a mirror which deflects it parallel to the monitor beamto spectrograph 140. It is understood that the paths from the beamsplitter and mirror to the respective spectrometers may include otheroptical components which are not shown in FIG. 1 but are, in a preferredembodiment, as similar as possible for the two beams. In the case wherethe sample is the sample under test, e.g., a wafer that has just beenpolished, the reflection spectrum is the measurement spectrum S, and itsassociated monitor spectrum is S_(m). The monitor spectrum is used tocorrect for rapid changes in the system, e.g., flickering of theillumination source.

FIG. 4 shows an exemplary embodiment of the alignment system allowingrapid alignment of a wafer with the optical system. In FIG. 4, wafer403, rotary chuck 402, motor 412, water 404, window 405, water level410, motor housing 400, rotary seal 401, light source 407, light 413,alignment window 408, detector 406, and tank wall 411 are shown.

In FIG. 4, rigid rotary chuck 402 holds wafer 403. Motor 412 turns therigid rotary chuck about an axis (not shown). Water 404 fills the areaabove main window 405 up to water level 410 and over to tank wall 411.Rotary seal 401 seals motor housing 400 from the water. Light source 407is also in a dry housing. The light source produces light 413 thatpasses through alignment window 408 from the dry housing into the water.Detector 406 is in the dry volume below window 405. Some of the light413 strikes wafer 403 and is blocked. The rest of the light passesthrough main window 405 into the dry volume below it, and onto thedetector.

Rigid rotary chuck 402 rotates wafer 403. As the wafer rotates, the edgeof the wafer that is directly over the detector moves in a radialdirection (to the left and right in FIG. 4). The radial motion arisesdue to the wafer being off-center on the rigid rotary chuck or not beingperfectly round. Aside from machining tolerances, the presence of afiducial notch or flat on the rigid rotary chuck causes the wafer to beout of round.

Radial motion of the edge of wafer 403 over detector 406 changes theshadowing of light 413 which falls upon the detector. The detector canbe either a single long detector, e.g., a photo-diode, or an array ofdetectors, e.g., a charge coupled device (CCD). In the former case, thetotal amount of the light falling on the detector is an indication ofposition of the edge of the wafer. As the edge of wafer 403 moves to theright in FIG. 4, the amount of light falling upon the detectordecreases. In general, the output of the detector, I, is some functionof the position of the edge of the wafer, Xe;I=f(Xe),  (1)

The quantity I is not necessarily linear but is monotonic, so that itsinverseXe=f ¹(I),  (2)

-   -   may be used to determine the location of the edge.

In an alternate embodiment, the detector may consist of an array ofdetectors, with each element in the array having a different location,Xa. In this case the intensity of light falling on the differentdetector elements gives rise to a waveform:I(Xa)=g(Xe).  (3)

The quantity I can be processed by an algorithm, h, such thatXe=h(I(Xa))  (4)

Functions g and f may be complicated, due to wave-optics considerations(FIG. 4 is illustrative only of ray-optics). Determination of f or g isby calibration.

FIG. 5 illustrates calibration of the wafer alignment. In FIG. 5, spiralwafer 500, chuck 503, spiral edges 504 and 505, detector 506, and source507 are shown.

Spiral wafer 500 has a thickness comparable to that of a silicon wafer;is made from a durable, clean, machinable, opaque material, eg,stainless steel; and has a mechanical index to insure that its center isaligned with the center of the chuck 503. As the chuck rotates, thespiral edges 504 and 505 block amounts of light emanating from source507 from reaching detector 506. As the spiral rotates, the systemrecords the detector output as a function of angle. The discontinuity inthe radius of the spiral 505 indicates when the spiral is over thedetector. The radius of the spiral as a function of angular displacementfrom the discontinuity 505 is known. Thus, the functions g(Xe) or f(Xe)can be recorded, so that f¹ or h can be calculated for use with realwafers.

The outcome of the above-described measurement enables the calculationof the location of a notch or flat on the wafer, and the location of thecenter of the wafer with respect to the center of the chuck, from I fora set of rotations covering 360 degrees with f¹ or h.

FIG. 6 shows another aspect of the wafer metrology device that improvesthe accuracy of wafer alignment. In FIG. 6, beam splitter 600, lens 601,reference detector 602, light source 607, window 608, rays 613,collimating lens 610, and wafer 606 are shown.

In general, the intensity of the source 607 can vary as a function of,eg, time and temperature. In order to correct or compensate for this,some portion of the light can be deflected by a beam splitter 600,possibly focused by lens 601, and detected by reference detector 602.The output from the reference detector can be used either to control theoutput intensity of the source, or to correct the inversion of data forvariations in the source.

FIG. 6 shows another exemplary illumination scheme. In this case thesource 607 produces diverging light. In this embodiment, lens 613collimates the rays 613. In other embodiments a collimated source may beused. Additional embodiments may uses a diffusing element following thesource in order to homogenize the spatial mode profile of the source.

There are three distinct advantages to wafer alignment: First, duringtraining the operator can always view the wafer “right-side-up,” e.g.,with the notch in the direction towards the bottom of the view screen.This makes training of the system easier. Second, pattern recognition ismore difficult with arbitrary orientations of the wafer. The better theinitial alignment, the easier is pattern recognition. Third, the pinholecan have a square cross section (perpendicular to the measurement ormonitor beams), which allows for greater light transmittance without anincrease in the minimum box size that can be used for the measurement.

Embodiments of this wafer metrology device may be integrated into awafer processor. As described above, different embodiments of this wafermetrology device allow it to be at different positions relative to thewafer under test. Particular embodiments utilize raiser and feederelements to take wafers from other locations and introduce them to anapparatus according to this wafer metrology device.

FIG. 9 shows a preferred embodiment integrated with a wafer processstation in a fabrication line. For purposes of illustration and notlimitation, the process station in the embodiment in FIG. 9 is apolisher. A polishing machine 1 and an integrated surface metrologystation, ISMS 10, are shown. The polishing machine 1 comprises apolishing unit 14, loading areas 18 and transport system 22. Inaddition, wafers 16 in carriers 18 are shown. As shown in FIG. 9, themetrology station is apart from the process station and coupled to theprocess station.

Wafers 16 are brought to and taken from polishing machine 1 in carriers18 through loading areas 20. The carriers may be cassettes or FOUPs,terms common in the art. Transport system 22 is a device or set ofdevices for transporting the wafers within polisher 1. Specificembodiments may comprise a robot, such as the EquipeWTM-105. Thetransport system can move the wafers to any of the carriers 18, thepolishing unit 14 or the ISMS 10.

The foregoing description of various embodiments of the invention hasbeen presented for purposes of illustration and description. It is notintended to limit the invention to the precise forms disclosed. Manymodifications and equivalent arrangements will be apparent.

1. A method of fabricating a semiconductor wafer, comprising the stepsof: transferring a processed wafer from a wafer process station to ametrology station spaced apart but coupled to the process station, themetrology station containing a rotatable chuck for receiving andsupporting the wafer and a translatable measurement head for measuringthe wafer; imaging a surface of the wafer with a camera in order tolocate a selected measurement region of the surface, the image beingindicative of at least one feature on the surface; adjusting a positionof an objective lens of the measurement head relative to the wafer to bealigned with the selected measurement region imaged by the camera, thetranslatable measurement head capable of translating the objective lensalong a linear axis parallel to a plane of the surface of the wafer;directing a broadband light beam toward the wafer using an optical fibercoupling a light source to the translatable measurement head; obtaininga first measurement of spectral content of the broadband light beamwhich has been reflected from the wafer through the positioned objectivelens; obtaining a second measurement of spectral content of thebroadband light beam which has not been reflected from the wafer; andreceiving the first and second measurements at a processor andevaluating the sample based on the first and second measurements, wherethe second measurement is used to correct for system characteristics. 2.A method according to claim 1, wherein: directing the broadband lightbeam toward the wafer includes using a beam splitter positioned along abeam path of the broadband light beam.
 3. A method according to claim 1,wherein: directing the broadband light beam toward the wafer includesusing a mirror positioned along a beam path of the broadband light beam.4. A method according to claim 1, further comprising: focusing thebroadband light beam on the sample using the objective lens of themeasurement head.
 5. A method according to claim 1, further comprising:loading the wafer into the wafer process station using a transportsystem.
 6. A method according to claim 5, further comprising: processingthe wafer in the process station.
 7. A method according to claim 1,wherein: the first and second measurements are obtained simultaneously.8. A method according to claim 1, wherein: the broadband light beam isgenerated by a UV light source.
 9. A method according to claim 1,wherein: the broadband light beam is generated by a light source definedby at least one lamp, said light source emitting a range of wavelengths,said range of wavelengths including visible and ultraviolet light.
 10. Amethod according to claim 1, wherein: the broadband light beam isgenerated by a lamp selected from the group consisting of a UV xenonlamp, a tungsten lamp, a deuterium lamp and a xenon lamp.
 11. A methodaccording to claim 1, further comprising: detecting an edge position ofthe wafer while the rotatable chuck is rotated in order to determine aposition offset of the sample.
 12. A method according to claim 1,further comprising: passing the broadband light beam, reflected from thewafer, through a pinhole mirror before obtaining the first measurement.13. A method according to claim 1, wherein: the position of theobjective lens relative to the wafer can be adjusted without altering anoptical path length of the metrology station.
 14. A method offabricating a semiconductor wafer, comprising the steps of: transferringa processed wafer from a wafer process station to a metrology stationspaced apart but coupled to the process station, the metrology stationcontaining a rotatable chuck for receiving and supporting the wafer anda translatable measurement head for measuring the wafer; rotating therotatable chuck to orient the wafer at a predetermined position;adjusting a position of the measurement head relative to the wafer to bealigned with a measurement location on the wafer; generating a broadbandlight beam using a light source that is separate from the measurementhead; directing the broadband light beam toward the wafer using anoptical fiber coupling the light source to the translatable measurementhead; passing the broadband light beam, reflected from the wafer,through a pinhole mirror before obtaining a first measurement; obtainingthe first measurement of spectral content of the broadband light beamwhich has been reflected from the wafer; obtaining a second measurementof spectral content of the broadband light beam which has not beenreflected from the wafer; and receiving the first and secondmeasurements at a processor and evaluating the sample based on the firstand second measurements, where the second measurement is used to correctfor system characteristics; receiving a reflected portion of thebroadband light beam, reflected by the pinhole mirror, to a camera fordetermining a measurement position relative to the wafer.
 15. A methodaccording to claim 14, further comprising: focusing the pinhole of thepinhole mirror onto the camera in order to determine a precisemeasurement position relative to the wafer.